This invention relates to treatment of a metal surface with an aqueous liquid composition that, before and/or during drying of the liquid composition into place on the surface, spontaneously reacts with the metal surface, without any application of electromotive force from an external source, to produce on the metal a surface with better corrosion resistance than the original untreated metal surface. More particularly, this invention is related to a process that provides a corrosion protective treatment that also provides lubricating qualities, so that mechanical deformation of the surface can be more readily accomplished, without unacceptable damage to the surface, than can the same mechanical deformation of the original untreated metal surface. Still more particularly, the metal surface treated is an alloy of zinc and aluminum, and/or the lubricating quality of the surface is sufficiently good that a metal sheet or coil having at least one surface modified by the treatment can be roll formed and/or press molded without unacceptable surface damage and without the use of any additional lubricant material on the part of the treated metal surface that is being mechanically deformed.
Aluminum-zinc alloy coated steel sheets are steel sheets which have been coated with an alloy which consists of 4-75% by weight of aluminum, the remainder being mainly zinc with traces of other components such as silicon, magnesium, cerium, lanthanum, or the like. At the +present time at least two such types of sheet steel are being manufactured: low aluminum-zinc alloy coated steel sheets, which are coated with an alloy consisting of 4-10% by weight of aluminum, the remainder consisting mainly of zinc with traces of magnesium, cerium, and/or lanthanum, and high aluminum-zinc alloy coated steel sheets, which are coated with an alloy consisting of 55% by weight of aluminum, 43.4% by weight of zinc, and 1.6% by weight of silicon. If the thickness of the coated layer on these sheets is the same as that on otherwise similar hot-dip galvanized steel sheets, the corrosion resistance is improved by a factor of some 1.5-2 times in the case of the low aluminum-zinc coated steel sheets and by a factor of some 3-6 times in the case of the high aluminum-zinc alloy coated steel sheets. Furthermore the high aluminum-zinc alloy coated steel sheets also have heat reflecting properties and excellent resistance to heat. As a result of their excellent properties, these aluminum-zinc alloy coated steel sheets have found wide application as building materials in the form of roofing and walling materials, in civil engineering applications, e.g., as guard rails, sound insulating barriers, anti-snow fencing, or drainage gullies, as materials for automobiles, domestic appliances, and industrial machinery and, after having been painted, as replacements for painted steel sheets.
However, the coated layer on these aluminum-zinc alloy coated steel sheets provides less effective lubrication than does the substantially pure zinc such as is found on the surface of hot-dip galvanized steel when roll forming or press molding these materials. The surface of aluminum-zinc alloy is more susceptible to damage; the coated layer may become fused onto the forming rolls or the pressing dies as a result of the heat which is generated by friction during such operations; and metal powder which has become detached from the aluminum-zinc alloy coated sheet steel may become attached to the forming rolls or the pressing dies. This powder may accumulate in the corner parts of a molded product, or it may cause defects that blemish the external appearance of the product.
In the past these problems have been overcome by coating the surface of the aluminum-zinc coated sheet steel with a lubricant such as oil or wax when carrying out shaping operations, in order to provide adequate lubrication at the surface of the aluminum-zinc alloy coated steel sheet. However, when such lubricating oils or waxes are used, there are disadvantages in subsequent processing and/or use. For example: the surface may be so slippery that an initially cylindrical coil of the finished material can readily be distorted by the force of gravity into a cone shape unless it is maintained strictly horizontal at all times when not mechanically restrained from such shape deformation; anything placed on an inclined surface of the coated lubricated material can more readily slip off than if the oil or wax were not present; and, if a coating is to be applied to the aluminum-zinc coated steel sheet after the shaping process, it is usually essential that all of the lubricating oil should be cleaned from the surface before applying any subsequent coating. Removing a lubricating oil or wax from the surface to a sufficient degree to assure uniformly good adhesion of the coating applied over it often is very difficult. Furthermore, when aluminum-zinc alloy coated steel sheet is shaped using a lubricating oil in the way described above, the lubricating oil becomes attached to the forming rolls or the pressing dies and if, at a later time, surface coated steel sheet is shaped using these same forming rolls or pressing dies, the lubricating oil often is transferred to the coated surfaces of these coated steel sheets, which are thereby stained. Any metal powder which is produced during the shaping process can become attached to the coated surface by way of the lubricating oil, so that it is necessary to clean the forming rolls or pressing dies thoroughly before carrying out such operations in order to prevent the occurrence of this type of contamination; such cleaning is very troublesome.
Additionally, the surfaces of aluminum-zinc alloy coated steel sheets are often subjected to a chromate treatment in order to prevent the formation of rust while the material is being stored. However if the aluminum-zinc alloy coated steel sheets have been treated with a water soluble lubricating oil, chromating treatment can lead to problems with pollution due to the dissolution of the chromate later.
U.S. Pat. No. 4,637,840 is believed to be the closest prior art. This teaches that a treatment composition consisting of water, hexavalent chromium, and a water soluble or water dispersible resin provides an external surface that does not need lubricating oil or wax. However, it has recently been found that the treatments provided according to the teachings of that patent are not always adequate to avoid a need for the use of lubricating oil to avoid surface damage during press and roll forming under current operating conditions. Accordingly, a major object of this invention is to provide a surface layer on metal substrates, particularly those of aluminum-zinc alloy, that will more reliably permit press and/or roll forming without the need for additional lubrication with materials that (i) diminish the adhesion or blemish the surface appearance of a subsequently applied coating and/or (ii) make the surface so slippery as to cause substantial practical difficulties in further processing or use of the metal with a surface treatment formed according to this invention. Other alternative and/or concurrent objects will be apparent from the description below.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout the description, unless expressly stated to the contrary: percent, “parts of”, and ratio values are by weight or mass; the term “polymer” includes “oligomer”, “copolymer”, “terpolymer” and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) noted in the specification between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added, and does not necessarily preclude unspecified chemical interactions among the constituents of a mixture once mixed; specification of constituents in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole and for any substance added to the composition; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to an object of the invention; the word “mole” means “gram mole”, and the word itself and all of its grammatical variations may be used for any chemical species defined by all of the types and numbers of atoms present in it, irrespective of whether the species is ionic, neutral, unstable, hypothetical, or in fact a stable neutral substance with well defined molecules; and the terms “solution”, “soluble”, “homogeneous”, and the like are to be understood as including not only true equilibrium solutions or homogeneity but also dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 100, or preferably at least 1000, hours during which the material is mechanically undisturbed and the temperature of the material is maintained within the range of 18-25° C.